In the field of the electrophotography, there have recently been developed a variety of copying apparatuses, which make use of various fixing processes. A fixing process which has presently been most commonly used is the heat fusing roll process. According to such process, toner powders to be used for photocopiers are typically blends of thermoplastic polymers or copolymers, colorants, charge control agents and flow agents and have a particle diameter of 5 to 30 micron. In principle the electrophotographic process starts by bringing a uniform electric charge onto a photoreceptor by corona treatment. An image of the original document is then projected on the photoreceptor band. Because the areas of the photoreceptor that are exposed to light become conductive, the photoreceptor losses its surface charge while the dark unexposed areas remain insulating and charged.
A monolayer of toner particles charged to the opposite polarity is then applied and attracted to the charged areas. According to one of the presently used embodiments these toner particles are taken up by a silicone rubber band of low hardness at 100.degree. C. and are transferred under pressure to the paper preheated to the same temperature, to avoid problems in the case that the paper is blocked and to clean the paper dust, a toner cleaner is also present.
A disadvantage of such a process embodiment is the heat conduction to the photoconductor which has to be cooled. Working at lower temperatures and saving of energy has become a strong aim.
Due to a required more and more increased copying speed along with an increase in the quantity of information to be processed in high speed copying apparatus, the heat of heat fusing-roll is transferred to copying paper in only a small quantity per unit area of the copying paper, while the paper takes a large quantity of heat away from the heat fusing-roll, which leads to an increase in the heat loss.
Therefore the surface temperature of the heat-fusing roll had to be decreased. On the other hand, modern household copying apparatuses are widely used, the size of the copying apparatus has been made more and more compact and the temperature as well as the pressure of the heat fusing roll has gradually been lowered or reduced.
As a result of the before mentioned speeding up and miniaturization of the copying apparatuses there was a need for toners having a powder stability until 50.degree. C. and showing a low viscosity at 60.degree. C., as the conventional toners exhibited only insufficient fixing ability at these desired lower temperatures.
It may be thought that the hereinbefore mentioned problem might be solved by reducing the molecular weight of the resin normally used as a binder and hence the viscosity of the resulting toner in its molten state to thus improve the flow properties and hence the fixing ability of the toner. However, another problem is formed in that the toner tends to adhere to the surface of the heat fusing roll and the adhered toner is again transferred to the subsequent copying paper (so called offset phenomenon). Moreover the strength of the binders is lowered because of the low molecular weight of the resin used. More specifically, images thermally fixed on the paper are damaged through bending and rubbing of the paper and as a result the toner adhered to the paper drops off to thus cause defects of the images.
Therefore, this method did not provide a toner having excellent fixing ability.
It will be appreciated that there has long been desired for the development of a toner composition which can be fixed at low temperature, has high resistance to mechanical breakage and does not cause any offset phenomenon.
It is an object of the present invention to provide a binder composition for toner compositions which can meet the hereinbefore mentioned requirements.
Although the inclusion of waxes in minor amounts in electrophotographic toner compositions was known, as appears e.g. from the European patent application no. 0 463,840 and U.S. Pat. Nos. 5,278,016 and 5,229,242, no binder compositions for electrophotography toners were actually provided, which could meet the hereinbefore discussed.
The European Patent Application No. 0 463 840 actually disclosed a toner composition for electrophotography, which comprised at least a binder and a coloring material as essential components, wherein the binder comprised a resin mixture, prepared by heating a resin solution, comprising a styrene resin and a block copolymer, which comprised blocks, derived from at least one member selected from ethylenic hydrocarbons and conjugated diene hydrocarbons and blocks derived from styrene, and/or a hydrogenated product thereof, in an amount ranging from 0.1 to 20 parts by weight per 100 parts by weight of the styrene resin and preferably from 0.1 to 10 parts by weight per 100 parts of binder, to remove the solvent. A low molecular weight wax having a softening point from 60.degree. to 180.degree. C. and preferably from 80.degree. to 160.degree. C. could be added to said toner composition in an amount of ranging from 0.5 to 20 parts by weight per 100 parts by weight of the binder and preferably from 0.5 to 10 parts by weight per 100 parts by weight of the binder.
In particular in pages 5, lines 20 up to 23 of this patent application there was clearly taught to a person skilled in the art, that amounts of low molecular wax exceeding 20 parts by weight per 100 parts by weight of binder composition should greatly affect the level of electrification and as a result the images obtained after copying operations should become unclear.
The U.S. Pat. No. 5,229,242 disclosed a toner composition consisting essentially of first resin particles, second cross linked resin particles, pigment, wax component particles, and a compatibilizer formed by a graft copolymer or styrene-ethylene/butylene block copolymer, a styrene-ethylene/propylene block copolymer or a styrene-ethylene/butylene-styrene block copolymer, and wherein said wax component is substantially retained in the toner composition. The wax had a weight average molecular weight of from 1000 to 10000 and more preferably it consisted of a polyolefin or a mixture of polyolefins. The wax was present in an amount of from 1 to 10 weight percent and preferably in an amount of from 3 to 7 weight percent.
U.S. Pat. No. 5,278,016 disclosed a toner composition comprising surface halogenated toner particles said toner particles each comprising a core of a low melt resin and pigment particles, and a protective shell formed over said core, said protective shell being formed by reacting a surface portion of said low melt resin with a halogen.
As low melt resin a block copolymer (A--B)n was proposed, wherein A represented a polymer block of a first monomer and B represented a block of a second monomer and n is in the range of from 2 to 100.
The toner composition could optionally contain in addition a microcrystalline wax.
As a result of extensive research and experimentation the intermediate binder composition, which could provide the desired toner compositions aimed at, could be surprisingly obtained.